The present invention relates to spectroscopic optical elements for separating light by different wavelengths and particularly to these optical elements using two-dimensional or three-dimensional periodic structures.
Increase in capacity of an optical fiber communication network has been demanded intensely with the rapid advance of popularization of the Internet in recent years. Development of wavelength division multiplexing (WDM) optical communication as a means for increasing the capacity has been advanced rapidly. In WDM optical communication, optically functional elements such as an optical demultiplexer, a filter and an isolator excellent in wavelength selectivity are required because various kinds of information are transmitted individually by light having slightly different wavelengths. It is a matter of course that mass production, miniaturization, integration, stability, etc. are strongly required of the functional elements.
An optical demultiplexer (or a spectroscopic device) is used for demultiplexing/detecting an optical signal multiplexed with a plurality of wavelengths artificially as in wavelength division multiplexing optical communication or for spectrally analyzing target light as in spectrometry. The optical demultiplexer needs spectroscopic elements such as a prism, a wavelength filter, and a diffraction grating. Particularly, the diffraction grating is a typical spectroscopic element. For example, a quartz or silicon substrate having a periodic micro prismatic structure formed in its surface is used as the diffraction grating. Diffracted light rays generated by the periodic micro prismatic structure interfere with one another, so that light having a specific wavelength emerges in a specific direction. This property is used for the spectroscopic element.
FIG. 23 shows an example of a spectroscopic optical system using a diffraction grating. Wavelength-multiplexed light rays 30 output from an optical fiber 21 are collimated to parallel light rays 31 by a collimator lens 22. The parallel light rays 31 are input into a diffraction grating 23 and output at output angles different in accordance with the wavelengths. The output light rays 32 pass through the collimator lens 22 again, so that a group of convergent light spots 40 are formed on an acceptance surface 24. If photo detectors such as photo diodes or end surfaces of optical fibers are disposed as acceptance units in the positions of the convergent light spots respectively, signal outputs separated by predetermined wavelengths can be obtained. In addition, if light input into the diffraction grating has a continuous spectrum, outputs spectrally discrete in accordance with the pitch of the acceptance units disposed on the acceptance surface can be obtained.
A reflection diffraction grating satisfies the expression (1):sin θi+sin θ0=mλ/d  (1)in which m is the order of diffraction of the diffraction grating, d is a grating constant, λ is a wavelength used, θi is the angle between input light (an optical axis 5 of an optical fiber) and a line normal to the surface in which the diffraction grating is formed, and θ0 is the angle between output light and the normal line.
When the wavelength λ is changed by Δλ while θi is kept constant, the positional change Δx of each light ray which reaches the acceptance surface, which is separated by a distance L from the diffraction grating is given by the following expression.Δx={Lm/(d·cos θ0)}·Δλ  (2)Accordingly, signals separated by wavelengths can be obtained if the acceptance units are arranged on the acceptance surface at intervals of a positional pitch calculated in accordance with a wavelength pitch by the aforementioned expression.
An output angle from the diffraction grating, however, has little dependence on wavelength. Assume the case where light, for example, having wavelengths arranged at intervals of 0.8 nm (equivalent to a frequency pitch of 100 GHz) in a 1.55 μm-wavelength band used in optical communication needs to be demultiplexed. When the order m of diffraction is 25 in the condition that the input angle θi is 71.5° whereas the output angle θ0 is 38.5°, the grating constant d of the diffraction grating is 24.7 μm on the basis of the expression (1). The change of the output angle obtained in accordance with the wavelength pitch of 0.8 nm in this system is only about 0.06°. If the light needs to be separably accepted by acceptance elements arranged at intervals of 50 μm, a distance L of 48 mm is required on the basis of the expression (2).
That is, generally, the positional change Δx of each light spot on the acceptance surface needs to be not smaller than the order of tens of μm because each acceptance unit has a predetermined size. Because m and d which are constants of the diffraction grating cannot be changed significantly, the distance L needs to be made large in order to obtain a necessary value of Δx in accordance with a small wavelength change Δλ. Hence, there is a problem that device size cannot but become large in order to improve the performance of the optical demultiplexer using the diffraction grating.
A two-dimensional or three-dimensional photonic crystal has been proposed as an element larger in wavelength dispersion than the diffraction grating. As will be described later, the direction of movement of light rays incident onto the two-dimensional photonic crystal is decided by a unique photonic band structure, so that very large angular dispersion can be generated even in a small wavelength difference as shown in FIG. 24A (so-called super-prism effect) The super-prism effect has been reported in the following paper.
Physical Review B, Vol.58, No.16, p.R10096, 1998
In the case of the two-dimensional photonic crystal 10, it is said that a light output surface 12 needs to be processed to have a suitable angle to form a prism as shown in FIG. 24A so that light flux largely dispersed in the inside can be taken out and used. This is because the angular dispersion effect is canceled by the light output surface 13 if the light input surface 15 is parallel to the light output surface 13 as shown in FIG. 24B as long as a means for taking out light while changing the direction of dispersed light by using a refracting phenomenon is used in the same manner as a related-art prism.
In the process for making the output surface obliquely to obtain a prism shape, the photonic crystal however must be sufficiently thick. There is also a disadvantage in terms of productivity because a high-grade processing technique is required. Moreover, in such a prism-shaped optical element, the diameter of light flux needs to be increased to a certain degree. An element having a size corresponding to the diameter of light flux is required. In the existing situation, however, a large two-dimensional photonic crystal (e.g., not smaller than 1 mm on a side) is of no practical use in terms of absorption and scattering in the inside as well as it is very difficult to produce such a large two-dimensional photonic crystal. On the other hand, if light flux is narrowed, the required size of the photonic crystal can be reduced. In this case, there is however a problem that wavelength resolving power is lowered because the light flux is spread by diffraction.